Elemental silicon in the diamond structure (d-Si) dominates the semiconductor industry, whereas a lesser known allotrope of silicon, the Si136 type II clathrate structure, has only recently attracted attention as a wide band gap (1.8 eV) semiconductor material. Experimental demonstration of a tunable, nearly direct band gap across the Si136—Gey alloy suggests the potential for photovoltaic and other optoelectronic applications. Silicon clathrates are also actively being investigated as anode materials for lithium ion batteries due to their high charge storage capacity.
Type I clathrates based on silicon have been demonstrated as thermoelectric materials and superconductors. These silicon clathrate structures are based on periodic arrangements of covalent silicon cages (Si20, Si24, Si28) as illustrated in FIG. 1. FIG. 1 (a) illustrates NaSi (monoclinic, space group C2/c) is thermally decomposed to form the silicon clathrates. FIG. 1 illustrates the unit cells and constitute a polyhedral of type I (illustrated in FIG. 1 (b)) and type II clathrates (illustrated in FIG. 1(c)). The silicon clathrates are composed of a covalently bound silicon framework surrounding by isolated sodium ions (which is illustrated in the center of each silicon clathrate cage). Guest atoms within these large cages have been found to ‘rattle’, thereby lowering the thermal conductivity of these clathrate materials. The interconnected nature of the cages that make up the clathrate structure also provides opportunities for gas transportation and storage.
Silicon clathrates have been synthesized in type I (A8Si46) and type II structures (A24Si136) with the guest atom A typically being a group I or II element (Na, K, Rb, Cs or Ba) or combinations thereof. These low density silicon frameworks can be endohedrally doped by the presence of a guest atom in the cage. The electropositive guest species donate electrons to the framework, leading to extremely high (˜1022 cm−3) free electron densities and metallic behavior. Achieving controllable guest occupancies is critical for semiconducting applications.
To date, there have been no reports of guest atom removal from the type I silicon clathrate structure. In the case of the type II structure, most of the guest atoms can be evacuated by heating the clathrate structure under vacuum over a period of several days. However, it is non-trivial to attain less than several thousand ppm guest concentration. Frequently, type II clathrate synthesis is carried out by the thermal decomposition of the corresponding Zintl monosilicide phase (e.g. NaSi) under vacuum. During thermal decomposition, it is hypothesized that cation sublimation leads to charge imbalance in the mono-silicide, which causes subsequent rearrangement and bonding of silicon atoms into the four-coordinate cage structures of the clathrate. During this process, the remaining cations are expected to template cage formation. In-situ measurements suggest there may be some topotactic character to this transformation. Other synthetic techniques such as ionic liquid synthesis, spark plasma sintering, and more recently, synthesis by vapor-phase intercalation of sodium or K into graphite have been investigated.
All of these techniques offer attractive routes to explore new inorganic crystal phases, however, simultaneous phase selectivity and scale-up have been challenging. The seminal work in controlling phase selectivity was conducted by Horie et al., Controlled Thermal Decomposition of NaSi to Derive Silicon Clathrate Compounds, J. Solid State Chem., 2009, 182, pp. 129-135, who developed initial trends in phase selection based on temperature and local sodium vapor pressure. Upon decomposition of 20 mg loads of NaSi precursor, Horie et al. found that clathrate phase selection (type I or II) was heavily dependent on temperature and vapor pressure. In Horie, the crucible was partially covered to increase the local sodium vapor pressure and was found to increase the phase fraction of type I clathrate. However, understanding the details of how sodium vapor pressure influences the phase selection is difficult as the local sodium vapor pressure depends heavily on temperature (bulk sodium diffusion, sublimation rate and sodium saturation pressure). In a recent publication Wagner, et al., Electrochemical Cycling of Sodium-Filled Silicon Clathrate, ChemElectroChem, Sep. 23, 2013, DOI: 10.1002/celc.201300104, it was reported that the conditions for the synthesis of high purity type II silicon clathrate as reported by Horie et al. was not reproducible. Along with large scale, phase selective synthesis it is extremely important to demonstrate techniques for reducing the guest concentration to achieve semiconducting type II silicon clathrates. Sodium concentrations as low as 3000 ppm (x=0.5; NaxSi136) has been achieved by repeated heating of the type II clathrate under vacuum a process that takes multiple days. Intermediate HCl washing of the powder was used to remove sodium products (e.g. oxides) which build up on the surface. Further reduction in sodium concentration has been achieved by reacting the low sodium type II silicon clathrate with iodine vapor to form NaI. Repeating this process several times resulted in a small quantity of type II silicon clathrate with sodium guest atom concentration of 35 ppm. Once a low (x<2) sodium content is achieved in the type II clathrate, it can be separated from the type I and d-Si impurities through centrifugation.